The present invention relates to a method and apparatus for detecting the voltage in a selected area of an object of interest by electrooptic conversion using an electrooptic material, the refractive index of which changes in response to the electric field created by said voltage. The present invention relates particularly to a method and apparatus capable of voltage detection with high time resolution and sensitivity.
Ultrafast transistors such as MODFETs (modulation doped field-effect transistors), superlattice photodetectors and high-speed integrated circuits operate on the picosecond order and techniques are available today by which the electric field (electric lines of force) originating from a voltage in a selected area of such ultrafast devices can be measured in a noncontact manner with picosecond time resolution and microvolt sensitivity. An example of these techniques is an electric field detecting apparatus that makes use of the Pockels effect and which detects the electric field in a selected area of an object of interest by electrooptic conversion with an electrooptic material, such as the crystal of LiTaO.sub.3, the refractive index of which changes in response to said electric field. In this apparatus, the electrooptic material is placed between a polarizer and an analyzer having crossed directions of polarization, and the change in electric field is detected as the change in the amount of transmitted light.
There are two types of electric field detectors that make use of the Pockles effect, one being an "electrode" type in which an electrode on a plate-like electrooptic material is connected to the electrode on the object to be measured, and the other being a "probe" type characterized by easy access of the electrooptic material in probe form to a selected site of measurement. The first "electrode" type is described in U.S. Pat. Nos. 4,603,293 and 4,618,819, European Patent Application Unexamined Publication No. 197,196 and IEEE Journal of Quantum Electronics, Vol. QE-22, No. 1, January, 1986, pp. 69-78. The second "probe" type is described in CLEO, 1987, pp. 352-353, and LLE Review, Vol. 32, July-September, 1987, pp. 158-163.
The optical probe used in the latter system, which is to be brought into proximity with the site of measurement, is described in CLEO, 1987, pp. 352-353 and shown in FIG. 12 attached hereto. The probe generally indicated by 10 comprises a silica support 12 tipped with a LiTaO.sub.3 crystal 14 in the form of a truncated four-sided pyramid, which in turn is provided on the bottom face with a full reflecting mirror 16 that is formed by evaporation of a multilayered dielectric film for reflecting a detecting light beam 18. An object to be measured 20, which is typically an integrated circuit, has a two-dimensional array of electrodes 22 and an electric field represented by electric lines of force is created on the surface of the circuit between adjacent electrodes 22. Hence, if the tip of the probe 10 is brought close to the object 20, the refractive index of the LiTaO.sub.3 crystal 14 changes, causing the light beam 18 to be modulated. The modulation is converted to the change in the amount of transmitted light by means of a polarizer and an analyzer, whereby the electric field created between electrodes 22 on the object 20 is detected.
Another example of the optical probe 10 is described in LLE Review, Vol. 32, July-September, 1987, pp. 158-163 and shown in FIG. 13 attached hereto. A light beam 18 launched into a LiTaO.sub.3 crystal 30 is reflected perfectly at three times by the surfaces so that it emerges from the crystal even without employment of a reflecting film on the underside of the crystal. In this modified version of probe 10, the refractive index of the crystal 30 is modulated by the electric field created parallel to the Z-axis in the vicinity of the bottom face of the crystal.
The LiTaO.sub.3 crystal used as the electrooptic material in the conventional optical probes has a relative dielectric constant .epsilon. of 40 which is greater than that of the air. Therefore, if this crystal is brought close to the object 20, the electric field E created by electrodes 22 will be altered since it is determined by the equation E=D/.epsilon. (D is the electric flux density).
This phenomenon is explained as follows. In the case of a transverse modulator, if the LiTaO.sub.3 crystal is absent, the object 20 will create an electric field as shown in FIG. 14. If the LiTaO.sub.3 crystal 32 is placed into the electric field to be measured, the electric field will change as shown in FIG. 15, so that some equipotential lines will avoid the crystal 32. As a result, the electric field created in the crystal 32 becomes smaller than when the crystal is absent. In other words, less of the voltage created on the object 20 will be applied to the crystal 32. Therefore, the conventional optical probes using the LiTaO.sub.3 crystal as an electrooptic material are incapable of efficient detection of the voltage on the object of interest and it has been difficult to enhance the sensitivity of voltage detection by these probes. The same problem has been encounted in a longitudinal modulator and the equipotential lines change in such a way that some of them avoid the crystal 32 as shown in FIG. 16.